fix: complete PTY terminal modes and Shift key input support

[inureyes]

Summary

This PR fixes critical issues preventing sudo password input from working correctly in interactive PTY sessions by completing terminal mode configuration and fixing Shift key input handling.

Changes

1. Complete Terminal Modes Configuration (RFC 4254)

Added all OpenSSH-compatible terminal mode settings for proper PTY behavior:

Control Characters:

• VEOL, VEOL2: End-of-line characters
• VSTART, VSTOP: Flow control (XON/XOFF)
• VREPRINT: Reprint current line (Ctrl+R)
• VWERASE: Erase word (Ctrl+W)
• VLNEXT: Literal next character (Ctrl+V)
• VDISCARD: Discard output (Ctrl+O)

Input Modes:

• IGNPAR, PARMRK, INPCK: Parity handling
• ISTRIP: 8th bit handling
• INLCR, IGNCR: Line ending conversions

Local Modes:

• NOFLSH: Flush behavior after signals
• TOSTOP: Background process output control
• PENDIN: Pending input behavior

Output Modes:

• OCRNL, ONOCR, ONLRET: Line ending handling

Control Modes:

• PARENB, PARODD: Parity configuration

2. Shift Key Input Bug Fix

Fixed key_event_to_bytes() function in src/pty/session.rs:

• Previous behavior: Only accepted KeyModifiers::NONE, blocking uppercase letters and special characters
• New behavior: Accepts characters with no modifiers OR SHIFT modifier
• Unchanged: CONTROL, ALT, META combinations still handled separately

Root Cause Analysis

The issue occurred because:

1. Incomplete terminal modes: PTY was configured with minimal modes, missing critical settings for interactive programs
2. Shift key rejection: Input handler rejected all characters with SHIFT modifier, preventing:
   • Uppercase letters (e.g., 'A' = Shift+a)
   • Special characters requiring Shift (e.g., '!' = Shift+1)

Impact

Before

• ❌ Passwords with uppercase letters failed on Linux remote hosts
• ❌ Passwords with special characters requiring Shift failed
• ⚠️ macOS localhost partially worked (simpler passwords without Shift)

After

• ✅ All password types work correctly
• ✅ Full uppercase and special character support
• ✅ Complete OpenSSH-compatible terminal mode configuration
• ✅ Proper behavior for sudo, passwd, and other interactive programs

Testing

Tested with:

• sudo -s with complex passwords (uppercase + special characters)
• passwd command for password changes
• Both macOS localhost and Linux remote hosts
• Various password complexity requirements

Related Issues

Closes #40

Files Changed

• src/pty/session.rs: Terminal modes configuration and Shift key input handling